Development of ion beam focusing technique using tapered capillary and its application to micro-PIXE analysis

Abstract

A technique to fabricate tapered glass capillary optics with various taper shapes has been developed and MeV ion focusing was tested with this novel focusing optics. The dependence of the MeV ion focusing ability of the tapered capillary optics on the capillary wall material was investigated by comparing tapered capillaries made of borosilicate glass and lead glass. The energy spectrum from the capillary-focused proton beams showed that the beam focusing ratio for the lead glass capillary was slightly higher than that for the borosilicate glass capillary, but the enhancement was much smaller than expected from the target atomic number dependence of the Rutherford scattering cross section. Three-dimensional Monte Carlo simulations were performed to examine the effect of the capillary wall shape as well as that of the wall material on the beam transmission. When a constant taper angle capillary was used, the beam-focusing ratio was improved by a factor of 2 compared to when the conventional tapered capillary having a convex inner wall was used. These results indicated that the probability of the scattered ion escaping from the capillary wall plays a predominant role in the MeV ion transmission in the tapered capillary optics. The effect of inner wall surface roughness on the beam transmission was also investigated by examining the energy spectra of protons forward scattered by flat glass targets having different surface roughness. A novel beam-sampling system using a rotating beam chopper has been developed and used to normalize the measured count of the scattered proton with respect to the incident proton flux. The energy spectrum of the scattered protons showed that the surface roughness might reduce the probability of proton scattering by the glass target. Finally, the elemental compositions in the surface layer of old and modern Sangkhalok potteries were investigated by the glass-capillary-based micro-PIXE analysis. By scanning the cross section of the piece of the pottery with a ϕ70-µm microbeam, the elemental distributions in the glaze and the paint layers were separately determined and clearly showed the differences in the surface layer between the old and the modern potteries. The two-dimensional mapping of elements was successfully demonstrated for the old Sangkhalok sample and its effectiveness was proved.